Method for using only two base stations for determining the position of a mobile subscriber in a CDMA cellular telephone system

ABSTRACT

A method for determining the position of a mobile station within a cellular telephone system having first and second base stations. A round trip signal propagation time between the first base station and the mobile station is measured at the first base station. A first signal is transmitted from the first base station to the mobile station, and a second signal is transmitted from the second base station to the mobile station. An arrival time difference representing a time interval between a first relative time when the first signal is received at the mobile station and a second relative time when the second signal is received at the mobile station is measured at the mobile station. The position of the mobile station is determined in accordance with the round trip propagation time and the arrival time difference. 
     A method for determining the position of a mobile station within a cellular telephone system having first and second base stations. A first round trip signal propagation time between the first base station and the mobile station is measured at the first base station. A second round trip signal propagation time between the second base station and the mobile station is measured at the second base station. The position of the mobile station is determined in accordance with the first and second round trip propagation times.

FIELD OF THE INVENTION

The present invention relates generally to cellular telephone systems.More specifically, the present invention relates to systems and methodsfor determining the geographical position of a mobile subscriber withina cellular telephone system. Still more particularly, the presentinvention relates to a method for locating a mobile subscriber within acode division multiple access (CDMA) cellular telephone system.

BACKGROUND OF THE INVENTION

There are several desirable reasons for having a service that candetermine the position of a mobile radio operating within a cellulartelephone system. For example, such a positioning service could be usedfor locating emergency callers (911) or children positioned within acellular system. Alternatively, such a positioning service could be usedfor locating vehicles as part of a dispatching or fleet monitoringsystem. Also, cellular system operators could use such a positioningservice in order to customize service parameters based on an accurateknowledge of mobile telephone location. Such customization couldinclude, for example, providing lower cost services for limited mobilitycustomers. A positioning service would also be of use in locating stolencellular phones and for investigating fraudulent use of cellularservices.

Methods for radio position determination make use of techniques formeasuring the propagation delay of a radio signal, which is assumed totravel in a straight line from a transmitter to a receiver at the speedof light. A radio delay measurement in combination with an anglemeasurement provided by a directive antenna is the fundamental principleof radar location. Radar location is frequently augmented by use of atransponder in the mobile vehicle, rather than relying entirely on thesignal reflected by the mobile vehicle.

Alternatively, a so-called trilateration system may be used for locatinga mobile radio. In a trilateration system, multiple time delaymeasurements are made using multiple transmitters and/or receivers. TheLoran system is an example of a location system which transmits a seriesof coded pulses from base stations at known and fixed locations to amobile receiver. The mobile receiver compares the times of arrival ofsignals from the different transmitters to determine hyperbolic lines ofpositions. Similarly, the Global Positioning System (GPS) providestransmission from a set of 24 earth orbiting satellites. Mobilereceivers can determine their position by using knowledge of thesatellites' locations and the time delay differences between signalsreceived from four or more satellites.

From the above examples, it can be seen that radio position locationsystems can be divided into two types, those which allow a mobile userto determine its own position, such as GPS, and those which allowanother party to determine the position of a mobile transponder such asradar systems. The system disclosed in the present application includeselements of both types, but primarily of the second type, where thefixed portion of a radio system wishes to determine the location of amobile radio unit positioned within the system. Except in the case ofpassive radar, such systems generally require the mobile radio unit totransmit a radio signal.

U.S. Pat. No. 5,126,748, entitled "Dual Satellite Navigation Method andSystem," discloses a method of radio location where the mobile terminalboth transmits and receives signals, thereby allowing round trip timingmeasurements defining circular lines of position to be performed usingfewer transmitter sites than required for the Loran and GPS systems inwhich the mobile terminals contain only receiving capability. In othersystems, the mobile terminal may contain only a transmitter and theremaining system elements perform direction finding or multiplereceptions of the signal from different locations to determine theposition. An example of this is the SARSAT system for locating downedaircraft. In this system, the downed aircraft transmits a signal on theinternational distress frequency 121.5 MHz (and 273 MHz). An earthorbiting satellite relays the signal to an earth terminal. As thesatellite passes overhead, the change in Doppler shift can be detectedand a line of position can be determined. Multiple overhead passes bythe same or similar satellites can determine a set of lines of position,the intersection of which determines the location of the downedaircraft.

It has long been known that direct sequence spread spectrum signals haveuseful properties for ranging and position location. Some of theearliest spread spectrum anti-jamming military communications systemsalso included an accurate ranging capability. GPS is, of course, basedon the use of direct sequence spread spectrum waveforms. In GPS, auser's receiver determines its position in four dimensional space-timeby making time difference measurements on the signals being receivedfrom four or more satellites in view. The satellites are positioned ininclined, 12 hour orbits and arranged so that most of the time in mostplaces, enough satellites will be in view with adequate geometry topermit accurate position calculations. The GPS system informs navigationterminals of current satellite ephemeris information which is requiredfor position calculations.

The Telecommunications Industry Association (TIA) in association withthe Electronic Industry Association (EIA) has developed and published anInterim Standard entitled "Mobile Station-Base Station CompatabilityStandard for Dual-Mode Wideband Spread Spectrum Cellular System," andreferred to as TIA/EIA/IS-95-A, May, 1995 (hereafter "the IS-95standard.") The IS-95 standard supports a code division multiple access(CDMA) cellular system which synchronizes the transmissions of all cellsin a cellular system using the GPS satellite downlink signals to updaterubidium clocks. Thus, in the IS-95 CDMA system, timing is transferredfrom the GPS system directly to the cellular system.

The IS-95 CDMA system can determine the location of a mobile station inthree dimensional space-time (time plus two dimensional positioning)provided that the mobile receiver is able to receive and track the pilotsignals of three neighboring base stations and is provided with accuratelocation information of the base stations. Likewise, if three IS-95 basestations are able to make timing measurements of a mobile's signal, thesystem can determine the location of the mobile station. The IS-95 CDMAsystem implements the universal frequency reuse principal, wherein allsectors and all cells in the system normally operate on the samefrequency. This universal frequency reuse principal is central to CDMA'sachievement of high system capacity. However, the implementation of theuniversal frequency reuse prinicpal in a CDMA system can make locating amobile station problematic in those instances where a mobile stationcomes close to a base station. In such instances, it may becomedifficult to achieve an adequate SNR when trying to receive theneighboring base stations. More particularly, transmissions from theneighboring base stations will be jammed by the close-by base station--aclassic near/far problem.

A power control system, as described in patents (give the QUALCOMM powercontrol patent numbers), is necessary to solve the near/far problem forthe mobile to base station communication link. As the mobile comes closeto one base station, it reduces its transmitter power accordingly so asto achieve a just adequate Eb/No at the closest base station. Thisresults in a lower Eb/No at the neighboring base stations, perhapsmaking it difficult to receive the mobile's signal at these locations.Thus, as a result of the power control system, neighboring base stationswill typically have difficulty measuring mobile signal timing when amobile unit is positioned near a close-by base station.

In the IS-95 CDMA system, the processing gain is nominally 21 dB. Thisis simply the ratio of the chip rate (1.2288 MHz) to the maximum datarate (9600 bps). At a point equidistant between two base stations, thetransmitter power needed for both base stations is about the same. Theresulting SNR at both base stations of the received mobile signal willlikely be more than adequate to obtain good timing measurements.However, when the mobile station moves to a point closer to one basestation than another, the transmitter power will be reduced. This willlower the received Eb/No at the further away base station. Themeasurement SNR can be raised by integrating over a longer time intervalthan a single bit time, effectively increasing the processing gain. Forexample, if the signal were to be integrated over one code repetition or32768 chips, the SNR is improved by 24 dB compared to the SNR at 9600bps because the processing gain becomes 45 dB (10*log 32768). If a 5 dBSNR is needed for good time tracking, then the signal at the far basestation can be 30 dB weaker than the close base station. This SNR orbetter can be achieved in about 90% of the cell area, assuming 4th powerpropagation. Thus, in 90% of the system's coverage area, the basestations will be typically be capable of time difference measurements insupport of positioning, provided that good base station geometry isavailable to obtain accurate positioning. The 10% of the cell area wheretime difference measurements between base stations is not available(with the above specified integration time) corresponds to the center ofthe cell area out to approximately 30% of the maximum cell radius. Thus,for base stations separated by 4 miles (2 mile cell radius) the radiusof the area where positioning cannot be done with the above bandwidthassumptions is about 1000 meters.

It should be noted that there are limitations as to the time ofintegration that might be employed due to Doppler considerations. Forexample, if a mobile is traveling at 60 mph on a line between two basestations, the differential Doppler is about 2×10-7. This amounts toabout 170 Hz in the 800 MHz cellular band. This is sufficient Doppler tomake integration over 32768 chips somewhat difficult. Thus, the aboveestimates should be taken as best case.

The basic method of mobile station receive only positioning is describedabove. In this method, the mobile must receive three or more cell pilotsignals from three or more nearby base stations and calculate timedifferences of arrival of the pilot signals. These arrival timedifferences allow hyperbolic lines of position to be determined, withthe mobile terminal's position being where these hyperbolic linesintersect. However, for the reasons explained above, when the mobile istoo close to a base station to obtain an adequate SNR on the two fartheraway cells, the required signal arrival time differences cannot beeasily measured, and therefore some other method must be utilized todetermine the position of the mobile station.

It is therefore an object of the present invention to provide a mobileradio positioning system, wherein the position of the mobile radio maydetermined if the mobile radio is positioned close-by to the closestbase station.

These and other objects and advantages of the invention will become morefully apparent from the description and claims which follow or may belearned by the practice of the invention.

SUMMARY OF THE INVENTION

The present invention is directed to a method for determining theposition of a mobile station within a cellular telephone system havingfirst and second base stations. A round trip signal propagation timebetween the first base station and the mobile station is measured at thefirst base station. A first signal is transmitted from the first basestation to the mobile station, and a second signal is transmitted fromthe second base station to the mobile station. An arrival timedifference representing a time interval between a first relative timewhen the first signal is received at the mobile station and a secondrelative time when the second signal is received at the mobile stationis measured at the mobile station. The position of the mobile station isdetermined in accordance with the round trip propagation time and thearrival time difference.

In accordance with a further aspect, the present invention is directedto a method for determining the position of a mobile station within acellular telephone system having first and second base stations. A firstround trip signal propagation time between the first base station andthe mobile station is measured at the first base station. A second roundtrip signal propagation time between the second base station and themobile station is measured at the second base station. The position ofthe mobile station is determined in accordance with the first and secondround trip propagation times.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the manner in which the above-recited and other advantagesand objects of the invention are obtained and can be appreciated, a moreparticular description of the invention briefly described above will berendered by reference to a specific embodiment thereof which isillustrated in the appended drawings. Understanding that these drawingsdepict only a typical embodiment of the invention and are not thereforeto be considered limiting of its scope, the invention and the presentlyunderstood best mode thereof will be described and explained withadditional specificity and detail through the use of the accompanyingdrawings.

FIGS. 1 and 1A show the operation of a mobile radio positioning systemwhere a mobile station is switched to a positioning channel and powertransmissions from the mobile station are temporarily increased in orderto allow timing measurements to be made between the mobile station andneighboring base stations, in accordance with a preferred embodiment ofthe present invention.

FIGS. 2 and 2A show the operation of a mobile radio positioning systemwhere power transmissions from the mobile station are temporarilyincreased in order to allow timing measurements to be made between themobile station and neighboring base stations, in accordance with apreferred embodiment of the present invention.

FIG. 3 shows the operation of a mobile radio positioning system where abase station having a "transmit-only" slave antenna is used fordetermining the mobile radio position, in accordance with a preferredembodiment of the present invention.

FIG. 4 shows the operation of a mobile radio positioning system where abase station having a "receive-only" slave antenna is used fordetermining the mobile radio position, in accordance with a preferredembodiment of the present invention.

FIGS. 5-7 show the operation of mobile radio positioning systems whereinonly two base stations are used for determining the position of a mobilestation, in accordance with a preferred embodiment of the presentinvention.

FIG. 8 shows a mobile radio positioning system that uses a base stationhaving a rotating transmitting beam antenna for determining the positionof the mobile radio, in accordance with a preferred embodiment of thepresent invention.

FIG. 9 shows a mobile radio positioning system that uses a base stationhaving a rotating receiving beam antenna for determining the position ofthe mobile radio, in accordance with a preferred embodiment of thepresent invention.

FIGS. 10 and 10A show the operation of a mobile radio positioning systemwherein each cell in the cellular system has an RF channel that isdedicated for positioning uses and unavailable for voice communication,in accordance with a preferred embodiment of the present invention.

FIG. 11 shows the operation of a mobile radio positioning system where abase station transmitter turns itself off during predetermined periodsto allow timing measurements to made between the mobile radio andneighboring base stations, in accordance with a preferred embodiment ofthe present invention.

FIG. 12 shows the operation of a mobile radio positioning system wherethe power of a mobile station is temporarily increased for a frame inorder to allow timing measurements to made between the mobile radio andneighboring base stations, in accordance with a preferred embodiment ofthe present invention.

FIG. 13 is a schematic overview of an exemplary CDMA cellular telephonesystem in accordance with the present invention.

FIG. 14 is a block diagram of the cell-site equipment as implemented inthe CDMA cellular telephone system.

FIG. 15 is a block diagram of the cell-site receiver.

FIG. 16 is a block diagram of the mobile unit telephone configured forCDMA communications in the CDMA cellular telephone system.

FIG. 17 is a block diagram of the mobile unit receiver.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIGS. 1-12 illustrate systems for positioning a mobile radio within acellular telephone system, in accordance with preferred embodiments ofthe present invention. The positioning systems illustrated in FIGS. 1-12are each preferably implemented as part of a cellular telephone systemthat uses spread spectrum modulation techniques for communicatingbetween mobile units and base stations in the cellular telephone system.Exemplary telephone systems having mobile radio units and base stationsthat utilize such spread spectrum modulation (or CDMA) techniques forcommunicating within a cellular telephone system are disclosed in U.S.Pat. No. 5,103,459 entitled "System and Method for Generating SignalWaveforms in a CDMA Cellular system" and U.S. Pat. No. 5,109,390entitled "Diversity Receiver in a Cellular Telephone System." Thecontents of U.S. Pat. Nos. 5,103,459 and 5,109,390 are herebyincorporated herein in their entirety by reference. Mobile radio unitsand base stations of the type disclosed in U.S. Pat. Nos. 5,103,459 and5,109,390 are shown in further detail inconnection with FIGS. 13-17 andwill be referred to hereafter as CDMA mobile stations and CDMA basestations, respectively.

Referring now to FIGS. 1 and 1A, there is shown the operation of amobile radio positioning system 100 where a CDMA mobile station isswitched to a positioning channel and power transmissions from the CDMAmobile station are temporarily increased in order to allow timingmeasurements to be made between the mobile station and neighboring CDMAbase stations, in accordance with a preferred embodiment of the presentinvention. Positioning system 100 is invoked initially at step 110 whena CDMA mobile station (or mobile radio) is in voice communication on anormal RF traffic channel with one or base stations in the cellularsystem. As explained above in the background, when the mobile station isoperating on a normal RF traffic channel, its power level is carefullycontrolled and maintained at the lowest possible level in order tomaintain a high traffic capacity. This low power level is sufficient toallow the mobile station to communicate on the normal RF traffic channelwith a closest-by base station (or first base station). When the mobilestation is in such communication with the closest-by base station, theclosest-by base station uses its transmitter and receiver to perform around trip time measurement which represents the time it takes for aradio signal to propagate from the closest-by base station to the mobilestation and then from the mobile station back to the closest-by basestation. More particularly, the base station transmitter has atransmission clock which supplies a transmission clock synchronizationsetting (or a relative transmission time) when a CDMA radio signal istransmitted by the base station. In addition, the base station receiverhas means for demodulating a CDMA signal received back from a mobilestation, and for determining a reception clock synchronization setting(or a relative reception time) associated with when such signal isreceived at the base station. In step 110, by comparing the differencebetween the transmission clock synchronization setting and the receptionclock synchronization setting, the base station is able to perform around trip time measurement which represents the time it takes for aradio signal to propagate from the base station to the mobile stationand then from the mobile station back to the base station. Bymultiplying this round trip time measurement by the signal propagationspeed (i.e., the speed of light), a relative distance between the mobilestation and the closest-by base station can be determined. The roundtrip time measurement places the mobile station on a circle having aradius equal to such relative distance and centered about the closest-bybase station.

Next, in step 120, the system attempts to perform a timing measurementbetween the mobile station and a neighboring base station (or secondbase station). In step 120, this measurement is attempted while themobile station is operating on the normal RF traffic channel at lowpower. The timing measurement made in step 120 may consist of a roundtrip signal propagation time measurement between the mobile station andthe second base station. Alternatively, the timing measurement which isattempted in step 120 may correspond to the time difference at which thesignal from the mobile station is respectively received at the first andsecond base stations. By multiplying such an arrival time difference bythe signal propagation speed (i.e., the speed of light), either ahyperbolic line of position for the mobile station between the first andsecond base station or a further circular line of position for themobile station can be determined. Next, in step 130, the system attemptsto determine the position of the mobile station based on the timingmeasurements made in steps 110 and 120. More particularly, the systemattempts to find intersections between the circular line of positiondetermined in step 110 and the circular (or hyperbolic) line of positiondetermined in step 120. If the system finds more than one suchintersection, the exact position of the mobile station may be resolvedby using a sector antenna at one of the base stations to select theintersection that represents the true position of the mobile station inthe cellular system. Alternatively, in the event that the system findsmore than one such intersection, a further arrival time differencemeasurement between one of the first or second base stations and a thirdbase station may be used to resolve the true position of the mobilestation.

If the system was unable to successfully perform any timing measurementin step 120 because, for example, the mobile radio station was operatingat a power level that was below the minimum power required for thesecond base station to properly receive the mobile station's signal,then processing proceeds to step 140 where the mobile radio station isswitched from the normal RF traffic channel to a special RF positioningchannel. This special RF positioning channel represents a normal CDMAchannel having the capacity to support voice communication, but which isseparate from the normal RF traffic channel used in steps 110-130. Thesame RF channel is preferably used for this special RF positioningchannel throughout every cell in the CDMA cellular system. Thereafter,in step 150, while the mobile station is operating on the positioningchannel, the power of the transmissions from the mobile station areincreased to their maximum possible power level. While the transmissionsfrom the mobile station are being made at this increased power level, atiming measurement is made in step 160 between the mobile station and aneighboring base station. The timing measurement made in step 160 isidentical to that made in step 120, except that in step 160 the timingmeasurement is made using a signal that has been transmitted from themobile unit at an increased power level. In steps 170 and 180, the powerof transmissions from the mobile station are decreased to their normallow level and the mobile station is switched back to the normal RFtraffic channel. The period of time between steps 150 and 170 duringwhich the mobile station is operating at its increased power levelshould be sufficient to allow the timing measurement made in step 160 tobe successfully completed, and this time period may be as short as theperiod of one voice frame in the signal transmitted from the mobilestation.

Finally, in step 190, the system determines the position of the mobilestation based on the timing measurements made in steps 110 and 160. Moreparticularly, the system find one or more intersections between thecircular line of position determined in step 110 and the circular (orhyperbolic) line of position determined in step 160. If the system findsmore than one such intersection, the exact position of the mobilestation may be resolved by using a sector antenna at one of the basestations to select the intersection that represents the true position ofthe mobile station in the cellular system. The process shown in FIGS. 1and 1A is preferably repeated periodically in order to maintain currentposition information on a mobile station as it moves within the cellularsystem. The process may be repeated, for example, at a time intervalequivalent to one out of every 100 voice frames in the signaltransmitted by the mobile station, or alternatively, every one to threeseconds.

It will be understood by those skilled in the art that the positioncalculations made in steps 130 and 190 may be performed either withinone or more base stations or within the cellular system's switchingcenter.

In the embodiment described above, the power of the transmissions fromthe mobile station are initially increased to their maximum possiblepower level in step 150. In an alternative preferred embodiment, thepower level of the mobile station may be gradually ramped-up at, forexample, 20 db intervals, until such time as the second base station cansuccessfully perform the timing measurements required by step 160.

In the preferred embodiment of the present invention, a map matchingtable is used in steps 140 and 190 to improve the accuracy of theposition determination made by the system. Since the timing measurementsmade by system 100 preferably correspond to signal propagation times (ordifferences in signal propagation times), positioning accuracy will bedegraded by poor geometry between the mobile station and the basestations or by bent signal propagation paths. A map matching table isformed by assuming that the mobile station will be within a vehicletraveling on a public road and then compensating for poor base stationgeometry and bent propagation paths that will result in positionalcalculation errors at various points in such roads. A preferred methodfor developing such a map matching table would be to be perform a surveyof an area by driving a mobile station along the various roads in thearea. While the mobile station is being driven around, the timingmeasurements described above are performed at various locations in thearea. In addition, at each such location, the actual position of themobile station is determined by using, for example, GPS, and this actualposition is stored as entry in the table along with the timingmeasurements performed at the location. The timing measurements made insteps 110, 120 and/or 160 are then compared to the timing measurementsstored in the table, and the entries from the table which have timingmeasurements that most closely match the timing measurements made insteps 110, 120 and/or 160 are selected. The position of the mobilestation is then determined by interpolating between the actual positionsstored in the table for each of the selected entries.

Finally, although system 100 as described above has been implemented aspart of a spread spectrum or CDMA cellular system, it will be understoodby those skilled in the art that the steps of system 100 may beimplemented in connection with other modulation systems such as, forexample, time division multiple access modulation systems, in order todetermine the position of mobile stations operating within such systems.

Referring now to FIGS. 2 and 2A, there is shown the operation of amobile radio positioning system 200 where power transmissions from themobile station are temporarily increased in order to allow timingmeasurements to be made between the mobile station and neighboring basestations, in accordance with a preferred embodiment of the presentinvention. System 200 functions substantially the same as system 100,except that in system 200 the mobile station is not switched to aseparate positioning channel before its power level is increased so asto allow timing measurements to made at a second neighboring basestation. Thus, steps 210, 220 and 230 correspond substantially to steps110, 120 and 130, respectively, and steps 240, 250, 260 and 270correspond substantially to steps 150, 160, 170 and 190 respectively.System 200 may have a disadvantage when compared against system 100because, in system 200, other mobile stations operating on the normal RFtraffic channel may suffer a frame error when the mobile station beingpositioned increases its power level between steps 240 and 260. However,CDMA systems are typically able to tolerate such an occasional frameerror.

Referring now to FIG. 3, there is shown the operation of a mobile radiopositioning system 300 where a base station having a "transmit-only"slave antenna is used for determining the mobile radio position, inaccordance with a preferred embodiment of the present invention. Insystem 300, a modified CDMA base station is used in place of the normalCDMA base station. In this modified base station, two or more (andpreferably three) transmit-only slave antennas are located proximate to(within approximately a few hundred feet of) the normal (or master) basestation antenna. In the case of a three sector CDMA base station, threetransmit-only slave antennas are preferably used, wherein each of theslave antennas is positioned in a different one of the three sectors.Each slave antenna has associated circuitry for transmitting CDMAsignals; this associated circuitry will substantially resemble thesignal transmission circuitry used for transmissions of CDMA signalsfrom the master base station antenna. In steps 305, 310, 315 and 320,first, second, third and fourth different CDMA signals (each of whichhas a separate preassigned Walsh code) are respectively transmitted fromthe first, second and third slave antennas and the master antenna at thebase station. The first, second, third and fourth signals aretransmitted on a common CDMA traffic channel. In the event that thefirst, second and third signals are transmitted from slave antennaspositioned in different sectors, the first, second and third signalswill also have different pn code phases corresponding to the sectorsfrom which such signals were transmitted. In steps 325, 330, 335 and340, the four signals transmitted in steps 305, 310, 315 and 320 arerespectively received by the mobile station. The mobile station hasmeans for simultaneously demodulating multiple signals having differentWalsh codes and different pn code phases, and for determining a clocksynchronization setting (or a relative reception time) associated witheach such signal. In step 345, by comparing the differences between theclock synchronization settings associated with the signals transmittedfrom the master antenna and the slave antennas, the mobile station isable to calculate arrival time differences corresponding to the relativetimes when the signals transmitted in steps 305, 310, 315 and 320 werereceived by the mobile station. Finally, in step 350, the arrival timedifferences for the signals transmitted in steps 305, 310, 315 and 320are used to calculate at least two hyperbolic lines of position. Thesystem then identifies one or more intersections between thesehyperbolic lines of position. If the system finds more than one suchintersection, the exact position of the mobile station may be resolvedby using a sector antenna at the base station to select the intersectionthat represents the true position of the mobile station in the cellularsystem.

The positional calculation made in step 350 may be performed either inthe mobile station, a base station or the system's switching center. Inthe event that the calculation is performed in the mobile station, thecoordinates of the base station master antenna and the slave antennaswill have to be transmitted to the mobile station before the mobilestation will be able to determine the hyperbolic lines of positionsdescribed above. Alternatively, if the calculation is to be performed inthe base station, the arrival time differences measured by the mobileunit will need to be transmitted to the base station before thepositional calculation can be made. In the preferred embodiment ofsystem 300, a map matching table (as described above) is used in step350 to improve the accuracy of the position determination made by thesystem.

Referring now to FIG. 4, there is show the operation of a mobile radiopositioning system 400 where a base station having a "receive-only"slave antenna is used for determining the mobile radio position, inaccordance with a preferred embodiment of the present invention. Insystem 400, a modified CDMA base station is used in place of the normalCDMA base station. In this modified base station, two or morereceive-only slave antennas are located proximate to (withinapproximately a few hundred feet of) the normal (or master) base stationantenna. Each slave antenna has associated circuitry for receiving CDMAsignals; this associated circuitry will substantially resemble thesignal reception circuitry used for receiving CDMA signals at the masterbase station antenna. In the case of a sectorized base station, it ispreferable to have a receive-only slave antenna positioned within eachsector. Thus, in the case of a three sector CDMA base station, threereceive-only slave antennas are preferably used, wherein each of theslave antennas is positioned in a different one of the three sectors. Inaddition to performing the positioning function described below, thesereceive-only slave antennas can also be used as diversity antennas atthe base station.

In step 410, the mobile station transmits a CDMA voice communicationsignal using the normal RF traffic channel. In steps 420, 430 and 440,the signal transmitted in step 410 is received at the base station bythe first and second slave antennas and the master antenna,respectively. The two slave antennas and the master antenna each havemeans for demodulating the CDMA signal transmitted from the mobilestation, and for determining a clock synchronization setting (or arelative reception time) associated with when the signal is received byeach such antenna. In step 450, by comparing the differences between theclock synchronization settings associated with the signal received atthe master antenna and the slave antennas, the base station is able tocalculate arrival time differences corresponding to the relative timeswhen the signal transmitted step 410 was received by the slave antennasand the master antenna at the base station. Finally, in step 460, thearrival time differences for the signal received in steps 420, 430 and440 are used to calculate two hyperbolic lines of position. The systemthen identifies one or more intersections between these hyperbolic linesof position. If the system finds more than one such intersection, theexact position of the mobile station may be resolved by using a sectorantenna at the base station to select the intersection that representsthe true position of the mobile station in the cellular system.

It will be understood by those skilled in the art that the positioncalculations made in step 460 may be performed either within the basestation or within the cellular system's switching center. In thepreferred embodiment of system 400, a map matching table (as describedabove) is used in steps 450 and 460 to improve the accuracy of theposition determination made by the system.

In an alternative embodiment of system 400, where the slave antennaswere unable to receive the signal in steps 420 and 430, because, forexample, the mobile radio station was operating at a power level thatwas below the minimum power required for the slave antennas to properlyreceive the mobile station's signal, then the power of the transmissionsfrom the mobile station may be temporarily increased to a higher powerlevel. In a preferred embodiment, this higher power level is achievedusing the closed loop power control system installed in the CDMA basestation. Typically, this power control system uses the signal receivedat the master base station antenna to adjust the power level of themobile station. However, in the event that one or more of the slaveantennas is unable to receive the mobile station in steps 420 and/or430, the power control system preferably changes its input so as to usethe weakest signal received at the slave antennas to adjust the powerlevel of the mobile station. This method guarantees that the signal fromthe mobile station will be increased to a power level that is sufficientfor reception at all the slave antennas. While the transmissions fromthe mobile station are being made at this increased power level, thetiming measurements made in steps 420, 430 and 440 are then performed.Thereafter, the power of transmissions from the mobile station isdecreased to the normal low level. As described above, the period oftime during which the mobile station is operating at its increased powerlevel should be sufficient to allow the timing measurements made in step420, 430 and 440 to be successfully completed, and this time period maybe as short as the period of one voice frame in the signal transmittedfrom the mobile station.

The process shown in FIGS. 3 and 4 are preferably repeated periodicallyin order to maintain current position information on a mobile station asit moves within the cellular system. Each process may be repeated, forexample, at a time interval equivalent to one out of every 100 voiceframes in the signal transmitted by the mobile station, oralternatively, every one to three seconds. Finally, although systems 300and 400 as described above have been implemented as part of a spreadspectrum or CDMA cellular system, it will be understood by those skilledin the art that the steps of these systems may be implemented inconnection with other modulation systems such as, for example, timedivision multiple access modulation systems, in order to determine theposition of mobile stations operating within such systems.

Referring now to FIG. 5, there is shown the operation of mobile radiopositioning system 500 wherein only two base stations are used fordetermining the position of a mobile station, in accordance with apreferred embodiment of the present invention. In steps 510 and 520, afirst CDMA signal having a first Walsh code and a first pn code offsetis transmitted from a first CDMA base station, and a second CDMA signalhaving a second (different) Walsh code and a second (different) pn codeoffset is transmitted from a second CDMA base station. The first andsecond signals are preferably transmitted on the normal RF trafficchannel used by the first and second base stations for communicatingwith mobile stations in their respective areas. In steps 515 and 520,the two signals transmitted in steps 510 and 520 are respectivelyreceived by a mobile station. The mobile station has means forsimultaneously demodulating multiple signals having different Walshcodes and different pn code offsets, and for determining a clocksynchronization setting (or a relative reception time) associated witheach such signal. In step 530, by comparing the differences between theclock synchronization settings associated with the signals transmittedfrom the first and second base stations, the mobile station is able tocalculate an arrival time difference corresponding to the relative timeswhen the two signals transmitted in steps 510 and 520 were received bythe mobile station. This arrival time difference will place the mobilestation on a hyperbolic line between the first and second base stations.Next, in step 530, the first base station will perform a round triptiming measurement between itself and the mobile station. As describedabove in connection with FIG. 1, such a round trip time measurementrepresents the time it takes for a radio signal to propagate from thefirst station to the mobile station and then from the mobile stationback to the first station. By multiplying this round trip timemeasurement by the signal propagation speed (i.e., the speed of light),a relative distance between the mobile station and the first basestation can be determined. The round trip time measurement thus placesthe mobile station on a circle having a radius equal to such relativedistance and centered about the first base station.

Next, in step 550, the system identifies one or more intersectionsbetween the hyperbolic and circular lines of position which weredetermined based on the measurements made in steps 530 and 540. Eachsuch intersection represents a candidate location where the mobilestation may be located. If the system finds more than one suchintersection, a sector antenna at one of the two base stations (oralternatively a sector antenna at a slave antenna) is used in step 560to determine the angular sector in which the mobile is located. In apreferred embodiment, the sector antennas will divide their receptionareas into three 120 degree sectors. If slave antennas are used in step560, the boundary lines between such sectors will preferably point atother slave antennas in the system. Finally, in step 570, the positionof the mobile station is determined by selecting the candidate locationpositioned within the sector identified in step 560. As described abovein connection with FIGS. 1-4, the positional calculations made in steps550 and 570 may be performed either within the mobile station or in oneof the base stations.

Referring now to FIG. 6, there is shown the operation of mobile radiopositioning system 600 wherein only two base stations are used fordetermining the position of a mobile station, in accordance with analternative preferred embodiment of the present invention. In step 610,a first base station performs a first round trip timing measurementbetween itself and the mobile station. As described above, this firstround trip time measurement places the mobile station on a first circlecentered about the first base station. Next, in step 620, a second basestation performs a further round trip timing measurement between itselfand the mobile station. This second round trip time measurement placesthe mobile station on a second circle centered about the second basestation.

Next, in step 630, the system identifies one or more intersectionsbetween the first and second circular lines of position which weredetermined based on the measurements made in steps 610 and 620. Eachsuch intersection represents a candidate location where the mobilestation may be located. If the system finds more than one suchintersection, a sector antenna at one of the two base stations (oralternatively a sector antenna at a slave antenna) is used in step 640to determine the angular sector in which the mobile is located. In apreferred embodiment, the sector antennas will divide their receptionareas into three 120 degree sectors. If slave antennas are used in step640, the boundary lines between such sectors will preferably point atother slave antennas in the system. Finally, in step 650, the positionof the mobile station is determined by selecting the candidate locationpositioned within the sector identified in step 640. As described abovein connection with FIG. 14, the positional calculations made in steps630 and 650 may be performed either within the mobile station or in oneof the base stations. In addition, a map matching table may be used toenhance the accuracy of the candidate locations identified in step 630.

Referring now to FIG. 7, there is shown the operation of mobile radiopositioning system 700 wherein only two base stations are used fordetermining the position of a mobile station, in accordance with a stillfurther alternative preferred embodiment of the present invention.System 700 is similar to system 600, except in system 700, if the firstand second base stations are unable to perform round trip timingmeasurements because the power level of the mobile station'stransmissions is too low, the power level of the mobile station'stransmissions is temporarily increased in order to allow such timingmeasurements to be made.

Referring still to FIG. 7, positioning system 700 is invoked initiallyat step 705 when a CDMA mobile station is in voice communication at lowpower on a normal RF traffic channel with one or base stations in thecellular system. This low power level is sufficient to allow the mobilestation to communicate on the normal RF traffic channel with aclosest-by base station (or first base station). In step 710, when themobile station is in such communication with the first base station, thefirst base station uses its transmitter and receiver to attempt toperform a round trip time measurement which represents the time it takesfor a radio signal to propagate from the first base station to themobile station and from the mobile station back to the first basestation. In step 720, while the mobile unit is still transmitting in itslow power mode, a neighboring base station (or second base station) usesits transmitter and receiver to attempt to perform a round trip timemeasurement which represents the time it takes for a radio signal topropagate from the second base station to the mobile station and fromthe mobile station back to the second base station. If the system isable to successfully perform the round trip timing measurements in steps710 and 715, processing proceeds to steps 745, 750 and 755, where theposition of the mobile station is determined based on such round triptiming measurements. Steps 745, 750 and 755 determine the position ofthe mobile station in substantially the same manner as steps 630, 640and 650, respectively, described above.

If system 700 was unable to successfully perform the timing measurementsin steps 705 and 710 because, for example, the mobile radio station wasoperating at a power level that was below the minimum power required forthe second base station to properly receive the mobile station's signal,then processing proceeds to step 720 where the power of thetransmissions from the mobile station are increased to their maximumpossible power level. While the transmissions from the mobile stationare being made at this increased power level, the timing measurementsthat were attempted originally in steps 705 and 710 are made in steps730 and 735. The timing measurements made in step 730 and 735 areidentical to those attempted in steps 705 and 710, except that in steps730 and 735 the timing measurements are made using a signal that hasbeen transmitted from the mobile unit at an increased power level.Thereafter, in step 740, the power of transmissions from the mobilestation are decreased to their normal low level, and the position of themobile station is determined in accordance with steps 745, 750 and 755as described above. In the preferred embodiment, the period of timebetween steps 720 and 740 during which the mobile station is operatingat its increased power level corresponds to the period of one voiceframe in the signal transmitted from the mobile station.

The process shown in FIG. 7 is preferably repeated periodically in orderto maintain current position information on a mobile station as it moveswithin the cellular system. The process may be repeated, for example, ata time interval equivalent to one out of every 100 voice frames in thesignal transmitted by the mobile station, or alternatively, every one tothree seconds. In addition, although systems 500, 600 and 700 asdescribed above have been implemented as part of a spread spectrum orCDMA cellular system, it will be understood by those skilled in the artthat the steps of these systems may be implemented in connection withother modulation systems such as, for example, time division multipleaccess modulation systems, in order to determine the position of mobilestations operating within such systems.

Referring now to FIG. 8, there is shown a mobile radio positioningsystem 800 that uses a CDMA base station 810 having a rotatingtransmitting beam antenna for determining the position of the mobilestation 820, in accordance with a preferred embodiment of the presentinvention. In system 800, a signal having its own Walsh code istransmitted from a rotating antenna at base station 810. The rotatingantenna has a beam 830 which rotates around a cell 840 in the cellulartelephone system. The beam rotates at, for example, one rotation everytwo seconds. In the event that the beam is rotating through varioussectors associated with base station 810, the pn code phase of thesignal transmitted from the rotating antenna will change to reflect thesector that the beam is rotating through. Thus, in the case of athree-sector base station, the pn code phase of the rotating beam signalwill change three times as the beam rotates one revolution around cell840. In an alternate embodiment, both the pn code phase and the Walshcode of the rotating beam signal will change as the beam rotates aroundcell 840. The beam 830 has a rotational timing that is known by themobile station 820. The mobile station receives this timing informationfrom signals transmitted by base station 810 to mobile station 820. Therotating beam signal is received at the mobile station 820, and based ona reception time when either a null or a peak of the rotating beamsignal is received by the mobile station 820, an angular displacementvalue (θ) corresponding to the angular position of the mobile station820 is determined. A first round trip signal propagation time between astationary antenna (preferably located at base station 810) and themobile station 820 is measured using a CDMA voice information signaltransmitted from the base station. The position of the mobile station isdetermined in accordance with the angular displacement value and thefirst round trip signal propagation time. More particularly, the roundtrip propagation time is used as described above to place the mobilestation 820 on a circle centered about the base station 810, and theangular displacement value (θ) is used to identify the point along thiscircle where mobile station 820 is located. This calculation may beperformed either in base station 810 or the cellular system's switchingcenter. A map matching table (as described above) may also be used toenhance the accuracy of the position determination made by system 800.

Referring now to FIG. 9, there is shown a mobile radio positioningsystem 900 that uses a base station 910 having a rotating receiving beamantenna for determining the position of the mobile station 920, inaccordance with a preferred embodiment of the present invention. Insystem 900, a CDMA voice information signal is transmitted from themobile station 920. The voice information signal is received at basestation 910 with a first antenna having a rotating beam 930 forreceiving the signal. Beam 930 rotates about cell 940 at a regularinterval. Based on a reception time when either a peak or null of thevoice information signal is received by the first antenna, an angulardisplacement value (θ) corresponding to the angular position of themobile station 920 is determined. A round trip signal propagation timebetween a second antenna (preferably located at base station 910) andthe mobile station 920 is measured. The position of the mobile station920 is then determined in accordance with the angular displacement value(θ) and the measured round trip signal propagation time. Moreparticularly, the round trip propagation time is used as to place themobile station 920 on a circle centered about the base station 910, andthe angular displacement value (θ) is used to identify the point alongthis circle where mobile station 920 is located. This calculation may beperformed either in base station 910 or the cellular system's switchingcenter. A map matching table (as described above) may also be used toenhance the accuracy of the position determination made by system 900.

Again, although systems 800 and 900 as described above have beenimplemented as part of a spread spectrum or CDMA cellular system, itwill be understood by those skilled in the art that the steps of thesesystems may be implemented in connection with other modulation systemssuch as, for example, time division multiple access modulation systems,in order to determine the position of mobile stations operating withinsuch systems.

Referring now to FIGS. 10 and 10A, there is shown the operation of amobile radio positioning system 1000 wherein each cell in the cellularsystem has an RF channel that is dedicated for positioning uses andunavailable for voice communication, in accordance with a preferredembodiment of the present invention. System 1000 is preferablyimplemented in connection with a CDMA cellular system in which each cellhas a plurality of N (where N is an integer greater two) RF trafficchannels, each of which has the capacity to support voice communicationsbetween a CDMA base station and a CDMA mobile station. In each cell, oneof the N traffic channels is designated as a dedicated positioningchannel that is normally unavailable for transmitting telephone voiceinformation signals to mobile stations within the cell. As a result ofthis designated positioning channel, the CDMA base station associatedwith each cell in the system will have N-1 normal RF traffic channelsthat are available to support voice communications between the basestation and a CDMA mobile station, and a single RF channel that is adedicated positioning channel that is unavailable for supporting suchvoice communications. In the preferred embodiment of the presentinvention, the dedicated positioning channels are selected for thevarious cells in the system such that neighboring cells have differentRF channels designated as their dedicated positioning channels.

Referring still to FIGS. 10 and 10A, system 1000 is initially invoked instep 1005 when a mobile station is communicating with a close-by basestation (or first base station) on one of the normal RF traffic channelsassociated with the first base station. When the mobile station is insuch communication with the first base station, the first base stationperforms a round trip time measurement which represents the time ittakes for a radio signal to propagate from the first base station to themobile station and then from the mobile station back to the first basestation. This round trip time measurement thus places the mobile stationon a circle centered about the first base station.

Next, in step 1010, the system attempts to perform a timing measurementbetween the mobile station and a neighboring base station (or secondbase station). In step 1010, this measurement is attempted while themobile station is operating on a normal RF traffic associated with thefirst base station. The timing measurement made in step 1010 may consistof a round trip signal propagation time measurement between the mobilestation and the second base station. Alternatively, the timingmeasurement which is attempted in step 1010 may correspond to the timedifference at which the signal from the mobile station is respectivelyreceived at the first and second base stations. In the event that thesystem was able to successful perform such timing measurements in step1010, processing proceeds to step 1035, where the system determines theposition of the mobile station based on the timing measurements made insteps 1005 and 1010. More particularly, the system identifies one ormore intersections between the circular line of position determined instep 1005 and the circular (or hyperbolic) line of position determinedin step 1010. If the system finds more than one such intersection, theexact position of the mobile station may be resolved by using a sectorantenna at one of the base stations to select the intersection thatrepresents the true position of the mobile station in the cellularsystem.

If system 1000 was unable to successfully perform any timing measurementin step 1010 because, for example, the mobile radio station wasoperating at a power level that was below the minimum power required forthe second base station to properly receive the mobile station's signal,then processing proceeds to step 1020 where the mobile radio station isswitched from a normal RF traffic channel to the dedicated RFpositioning channel associated with the first base station. While themobile station is operating on this dedicated RF positioning channel,the mobile station can clearly receive transmissions from neighboringbase stations. In step 1025, while the mobile station is on thededicated positioning channel and able to hear such neighboring basestations, the mobile station measures an arrival time difference ofsignals transmitted from neighboring base stations (or, alternatively,an arrival time difference between a signal transmitted from aneighboring base station and a signal transmitted from the first basestation). As described above, this arrival time difference, togetherwith the coordinates of the appropriate base stations, can be used toplace the mobile station on a hyperbola between such base stations. Insteps 1030, the mobile station is switched back to a normal RF trafficchannel. Finally, in step 1035 (the operation of which is describedabove), the system determines the position of the mobile station basedon the timing measurements made in steps 1005 and 1025. The positioncalculations made in step 1035 may be performed either within one ormore base stations or within the cellular system's switching center.

Referring now to FIG. 11, there is shown the operation of a mobile radiopositioning system 1100 where a base station transmitter turns itselfoff during predetermined periods to allow timing measurements to madebetween the mobile radio and neighboring base stations, in accordancewith a preferred embodiment of the present invention. System 1100 beginsat step 1110, when a first CDMA base station is in normal voicecommunication with a CDMA mobile station in the coverage area of thefirst base station. Next, in step 1120, while the first base stationcontinues to transmit to mobile stations within its coverage area, amobile station being positioned attempts to locate itself usingtrilateration, i.e, by attempting to measure signal arrival timedifferences between the first base station and two other neighboringbase stations. Such positioning will be unsuccessful if the mobilestation being positioned cannot make the required timing measurementswith neighboring base stations. In the event such positioning isunsuccessful, processing proceeds to step 1130, where the first basestation turns off its transmitter for a single vocoder frame. While thefirst base station's transmitter is silent, the mobile station beingpositioned measures arrival time differences of signals received from atleast three neighboring base stations in step 1140. In addition, in step1160, while the first base station's transmitter is silent, other mobilestations within the coverage area of the first base station mask anytransmission errors caused by the temporary interruption oftransmissions from the first base station transmitter for a vocoderframe. Next, in step 1150, the system determines the location of themobile station being positioned based on the timing measurements made instep 1140. More particularly, the system identifies one or moreintersections between hyperbolic lines of position defined by the timingmeasurements made in step 1140. If the system finds more than one suchintersection, the exact position of the mobile station may be resolvedby using a sector antenna at one of the base stations to select theintersection that represents the true position of the mobile station inthe cellular system. The position calculation performed in step 1150 maybe made either in the mobile station being positioned or in a basestation. Moreover, a map matching table may be used as described aboveto enhance the accuracy of the mobile position determination made instep 1150. After the position of the mobile station is determined instep 1150, transmissions are resumed from the first base station tomobile stations within the coverage area of the first base station.

The process shown in FIG. 11 is preferably repeated periodically inorder to maintain current position information on a mobile station as itmoves within the cellular system. The process may be repeated, forexample, at a time interval equivalent to one out of every 100 voiceframes in the signal transmitted by the first base station, oralternatively, every one to three seconds. In addition, the time periodsat which neighboring base stations cease transmissions in step 1130 arepreferably gated such that adjacent base stations do not ceasetransmissions simultaneously. Finally, although system 1100 as describedabove is preferably implemented as part of a spread spectrum or CDMAcellular system, it will be understood by those skilled in the art thatthe steps of these systems may be implemented in connection with othermodulation systems such as, for example, time division multiple accessmodulation systems, in order to determine the position of mobilestations operating within such systems.

Referring now to FIG. 12, there is shown the operation of a mobile radiopositioning system 1200 where the power of a mobile station istemporarily increased for a frame in order to allow timing measurementsto made between the mobile radio and neighboring base stations, inaccordance with a preferred embodiment of the present invention. System1200 begins at step 1210, when a first CDMA base station is in normalvoice communication at a low power level with a CDMA mobile station inthe coverage area of the first base station. Next, in step 1220, whilethe first base station continues to transmit to mobile stations withinits coverage area, a mobile station being positioned attempts to locateitself using trilateration, i.e, by attempting to measure signal arrivaltime differences between the first base station and two otherneighboring base stations. Step 1220 is substantially the same as step1120 described in connection with FIG. 11 above. In the event suchpositioning is unsuccessful, processing proceeds to step 1230, where theCDMA mobile station being positioned increases its transmission powerlevel to a maximum level for a single frame. In step 1240, while themobile station's transmitter is at maximum power, at least threeneighboring base stations measure arrival time differences of the signaltransmitted from the mobile station at maximum power. In addition, instep 1260, while the mobile station's transmitter is at maximum power,other mobile stations operating at low power within the same cell as themobile station being positioned mask any errors caused by the temporaryincrease in transmission power at the mobile station being positioned.Next, in step 1250, the system determines the location of the mobilestation being positioned based on the timing measurements made in step1240. More particularly, the system identifies one or more intersectionsbetween hyperbolic lines of position defined by the timing measurementsmade in step 1240. If the system finds more than one such intersection,the exact position of the mobile station may be resolved by using asector antenna at one of the base stations to select the intersectionthat represents the true position of the mobile station in the cellularsystem. The position calculation performed in step 1250 may be madeeither in the mobile station being positioned or in a base station.Moreover, a map matching table may be used as described above to enhancethe accuracy of the mobile position determination made in step 1250.After the position of the mobile station is determined in step 1250,transmissions are resumed at low power from the mobile station beingpositioned.

The process shown in FIG. 12 is preferably repeated periodically inorder to maintain current position information on a mobile station as itmoves within the cellular system. The process may be repeated, forexample, at a time interval equivalent to one out of every 100 voiceframes in the signal transmitted by the mobile station being positioned,or alternatively, every one to three seconds. In addition, althoughsystem 1200 as described above is preferably implemented as part of aspread spectrum or CDMA cellular system, it will be understood by thoseskilled in the art that the steps of these systems may be implemented inconnection with other modulation systems such as, for example, timedivision multiple access modulation systems, in order to determine theposition of mobile stations operating within such systems.

General Operation of CDMA Base Stations and Mobile Units (FIGS. 13-17)

In a CDMA cellular telephone system, each cell-site has plurality ofmodulator-demodulator units or spread spectrum modems. Each modemconsists of a digital spread spectrum transmit modulator, at least onedigital spread spectrum data receiver and a searcher receiver. Eachmodem at the cell-site is assigned to a mobile unit as needed tofacilitate communications with the assigned mobile unit.

In the CDMA cellular telephone system, each cell-site transmits a "pilotcarrier" signal. Should the cell be divided into sectors, each sectorhas an associated distinct pilot signal within the cell. This pilotsignal is used by the mobile units to obtain initial systemsynchronization and to provide robust time, frequency and phase trackingof the cell-site transmitted signals. Each cell-site also transmitsspread spectrum modulated information, such as cell-site identification,system timing, mobile paging information and various other controlsignals.

The pilot signal transmitted by each sector of each cell is of the samespreading code but with a different code phase offset. Phase offsetallows the pilot signals to be distinguished from one another thusdistinguishing originating cell-sites or sectors. Use of the same pilotsignal code allows the mobile unit to find system timing synchronizationby a single search through all pilot signal code phases. The strongestpilot signal as determined by a correlation process for each code phase,is readily identifiable. The identified strongest pilot signal generallycorresponds to the pilot signal transmitted by the nearest cell-site.However, the strongest pilot signal is used whether or not it istransmitted by the closest cell-site.

Upon acquisition of the strongest pilot signal, i.e., initialsynchronization of the mobile unit with the strongest pilot signal, themobile unit searches for another carrier intended to be received by allsystem users in the cell. This carrier, called the synchronizationchannel, transmits a broadcast message containing system information foruse by the mobiles in the system. The system information identifies thecell-site and the system in addition to conveying information whichallows the long PN codes, interleaver frames, vocoders and other systemtiming information used by the mobile unit to be synchronized withoutadditional searching. Another channel, called the paging channel mayalso be provided to transmit messages to mobiles indicating that a callhas arrived for them, and to respond with channel assignments when amobile initiates a call.

An exemplary telephone system in which the present invention is embodiedis illustrated in FIG. 13. The system illustrated in FIG. 13 utilizesCDMA modulation techniques in communication between the system mobileunits or mobile telephones, and the cell-sites. Cellular systems inlarge cities may have hundreds of cell-site stations serving hundreds ofthousands of mobile telephones. The use of spread spectrum techniques,in particular CDMA, readily facilitates increases in user capacity insystems of this size as compared to conventional FM modulation cellularsystems.

In FIG. 13, system controller and switch 10, also referred to as mobiletelephone switching office (MTSO), typically includes interface andprocessing circuitry for providing system control to the cell-sites.Controller 10 also controls the routing of telephone calls from thepublic switched telephone network (PSTN) to the appropriate cell-sitefor transmission to the appropriate mobile unit. Controller 10 alsocontrols the routing of calls from the mobile units, via at least onecell-site to the PSTN. Controller 10 may connect calls between mobileusers via the appropriate cell-sites since the mobile units do nottypically communicate directly with one another.

Controller 10 may be coupled to the cell-sites by various means such asdedicated telephone lines, optical fiber links or microwavecommunication links. In FIG. 13, two such exemplary cell-sites 12 and 14including, along with mobile units 16 and 18 each including a cellulartelephone are illustrated. Cell-sites 12 and 14 as discussed herein andillustrated in the drawings are considered to service an entire cell.However it should be understood that the cell may be geographicallydivided into sectors with each sector treated as a different coveragearea. Accordingly, handoffs are made between sectors of a same cell asis described herein for multiple cells, while diversity may also beachieved between sectors as is for cells.

In FIG. 13, arrowed lines 20a-20b and 22a-22b respectively define thepossible communication links between cell-site 12 and mobile unit 16 and18. Similarly, arrowed lines 24a-24b and 26a-26b respectively define thepossible communication links between cell-site 14 and mobile units 16and 18. Cell-sites 12 and 14 nominally transmit using equal power.

All the cells in a service area are supplied with accuratesynchronization. In the preferred embodiment, a GPS receiver at eachcell synchronizes the local waveform timing to Universal CoordinatedTime (UTC). The GPS system allows time synchronization to better than 1microsecond accuracy. Accurate synchronization of cells is desirable inorder to allow easy handoff of calls between cells when mobiles movefrom one cell to another with a call in progress. If the neighboringcells are synchronized, the mobile unit will not have difficultysynchronizing to the new cell thereby facilitating a smooth handoff.

The pilot carrier is transmitted at a higher power level than a typicalvoice carrier so as to provide greater signal to noise and interferencemargin for this signal. The higher power level pilot carrier enables theinitial acquisition search to be done at high speed and to make possiblea very accurate tracking of the carrier phase of the pilot carrier by arelatively wide bandwidth phase tracking circuit. The carrier phaseobtained from tracking the pilot carrier is used as the carrier phasereference for demodulation of the carriers modulated by user informationsignals. This technique allows many user carriers to share the commonpilot signal for carrier phase reference. For example, in a systemtransmitting a total of fifteen simultaneous voice carriers, the pilotcarrier might be allocated a transmit power equal to four voicecarriers.

In addition to the pilot carrier, another carrier intended to bereceived by all system users in the cell is transmitted by thecell-site. This carrier, called the synchronization channel, also usesthe same 32,768 length PN sequence for spectrum spreading but with adifferent, pre-assigned Walsh sequence. The synchronization channeltransmits a broadcast message containing system information for use bythe mobiles in the system. The system information identifies thecell-site and the system and conveys information allowing the long PNcodes used for mobile information signals to be synchronized withoutadditional searching.

Another channel, called the paging channel may be provided to transmitmessages to mobiles indicating that a call has arrived for them, and torespond with channel assignments when a mobile initiates a call.

Each voice carrier transmits a digital representation of the speech fora telephone call. The analog speech waveform is digitized using standarddigital telephone techniques and then compressed using a vocodingprocess to a data rate of approximately 9600 bits per second. This datasignal is then rate r=1/2, constraint length K=9 convolutional encoded,with repetition, and interleaved in order to provide error detection andcorrection functions which allow the system to operate at a much lowersignal-to-noise and interference ratio. Techniques for convolutionalencoding, repetition and interleaving are well known in the art.

The resulting encoded symbols are multiplied by an assigned Walshsequence and then multiplied by the outer PN code. This process resultsin a PN sequence rate of 1.2288 MHz or 128 times the 9600 bps data rate.The resulting signal is then modulated onto an RF carrier and summedwith the pilot and setup carriers, along with the other voice carriers.Summation may be accomplished at several different points in theprocessing such as at the IF frequency, or at the baseband frequencyeither before or after multiplication by the PN sequence.

Each voice carrier is also multiplied by a value that sets itstransmitted power relative to the power of the other voice carriers.This power control feature allows power to be allocated to those linksthat require higher power due to the intended recipient being in arelatively unfavoring location. Means are provided for the mobiles toreport their received signal-to-noise ratio to allow the power to be setat a level so as to provide adequate performance without waste. Theorthogonality property of the Walsh functions is not disturbed by usingdifferent power levels for the different voice carriers provided thattime alignment is maintained.

FIG. 14 illustrates in block diagram form an exemplary embodiment of thecell-site equipment. At the cell-site, two receiver systems are utilizedwith each having a separate antenna and analog receiver for spacediversity reception. In each of the receiver systems the signals areprocessed identically until the signals undergo a diversity combinationprocess. The elements within the dashed lines correspond to elementscorresponding to the communications between the cell-site and one mobileunit. The output of the analog receivers are also provided to otherelements used in communications with other mobile units.

In FIG. 14, the first receiver system is comprised of antenna 30, analogreceiver 32, searcher receiver 34 and digital data receiver 36. Thefirst receiver system may also include an optional digital data receiver38. The second receiver system includes antenna 40, analog receiver 42,searcher receiver 44 and digital data receiver 46.

The cell-site also includes cell-site control processor 48. Controlprocessor 48 is coupled to data receivers 36, 38 and 46 along withsearcher receivers 34 and 44. Control processor 48 provides among otherfunctions, functions such as signal processing; timing signalgeneration; power control; and control over handoff, diversity,diversity combining and system control processor interface with theMTSO. Walsh sequence assignment along with transmitter and receiverassignment is also provided by control processor 48.

Both receiver systems are coupled by data receivers 36, 38 and 46 todiversity combiner and decoder circuitry 50. Digital link 52 is coupledto receive the output of diversity combiner and decoder circuitry 50.Digital link 52 is also coupled to control processor 48, cell-sitetransmit modulator 54 and the MTSO digital switch. Digital link 52 isutilized to communicate signals to and from the MTSO with cell-sitetransmit modulator 54 and circuitry 50 under the control of controlprocessor 48.

The mobile unit transmitted signals are direct sequence spread spectrumsignals that are modulated by a PN sequence clocked at a predeterminedrate, which in the preferred embodiment is 1.2288 MHz. This clock rateis chosen to be an integer multiple of the baseband data rate of 9.6Kbps.

Signals received on antenna 30 are provided to analog receiver 32. Thedetails of receiver 32 are further illustrated in FIG. 15. Signalsreceived on antenna 30 are provided to downconverter 101 which iscomprised of RF amplifier 102 and mixer 104. The received signals areprovided as an input to RF amplifier where they are amplified and outputto an input to mixer 104. Mixer 104 is provided another input, thatbeing the output from frequency synthesizer 106. The amplified RFsignals are translated in mixer 104 to an IF frequency by mixing withthe frequency synthesizer output signal.

The IF signals are then output from mixer 104 to bandpass filter (BPF)108, typically a Surface Acoustic Wave (SAW) filter having a passband of1.25 MHz, where they are bandpass filtered. The filtered signals areoutput from BPF 108 to IF amplifier 110 where the signals are amplified.The amplified IF signals are output from IF amplifier 110 to analog todigital (A/D) converter 112 where they are digitized at a 9.8304 MHzclock rate which is exactly 8 times the PN chip rate. Although (A/D)converter 112 is illustrated as part of receiver 32, it could instead bea part of the data and searcher receivers. The digitized IF signals areoutput from (A/D) converter 112 to data receiver 36, optional datareceiver 38 and searcher receiver 34. The signals output from receiver32 and I and Q channel signals as discussed later. Although asillustrated in FIG. 3 with A/D converter 112 being a single device, withlater splitting of the I and Q channel signals, it is envisioned thatchannel splitting may be done prior to digitizing with two separate A/Dconverters provided for digitizing the I and Q channels. Schemes for theRF-IF-Baseband frequency downconversion and analog to digital conversionfor I and Q channels are well known in the art.

Searcher receiver 34 is used to at the cell-site to scan the time domainabout the received signal to ensure that the associated digital datareceiver 36, and data receiver 38 if used, are tracking and processingthe strongest available time domain signal. Searcher receiver 64provides a signal to cell-site control processor 48 which providescontrol signals to digital data receivers 36 and 38 for selecting theappropriate received signal for processing.

The signal processing in the cell-site data receivers and searcherreceiver is different in several aspects than the signal processing bysimilar elements in the mobile unit. In the inbound, i.e., reverse ormobile-to-cell link, the mobile unit does not transmit a pilot signalthat can be used for coherent reference purposes in signal processing atthe cell-site. The mobile-to-cell link is characterized by anon-coherent modulation and demodulation scheme using 64-ary orthogonalsignaling.

In the 64-ary orthogonal signaling process, the mobile unit transmittedsymbols are encoded into one of 2⁶, i.e., 64, different binarysequences. The set of sequences chosen are known as Walsh functions. Theoptimum receive function for the Walsh function m-ary signal encoding isthe Fast Hadamard Transform (FHT).

Referring again to FIG. 14, searcher receiver 34 and digital datareceivers 36 and 38, receive the signals output from analog receiver 32.In order to decode the spread spectrum signals transmitted to theparticular cell-site receiver through which the mobile unitcommunicates, the proper PN sequences must be generated. Further detailson the generation of the mobile unit signals are discussed later herein.

As illustrated in FIG. 15, receiver 36 includes two PN generators, PNgenerators 120 and 122, which generate two different short code PNsequences of the same length. These two PN sequences are common to thoseof all cell-site receivers and all mobile units with respect to theouter code of the modulation scheme as discussed in further detail laterherein. PN generators 120 and 122 thus respectively provide the outputsequences, PN_(I) and PN_(Q). The PN_(I) and PN_(Q) sequences arerespectively referred to as the In-Phase (I) and Quadrature (Q) channelPN sequences.

The two PN sequences, PN_(I) and PN_(Q) are generated by differentpolynomials of degree 15, augmented to produce sequences of length32,768 rather than 32,767 which would normally be produced. For example,the augmentation may appear in the form of the addition of a single zeroto the run of fourteen 0's in a row which appears one time in everymaximal-length linear sequence of degree 15. In other words, one stateof the PN generator would be repeated in the generation of the sequence.Thus, the modified sequence contains one run of fifteen 1's and one runof fifteen 0's.

In the exemplary embodiment receiver 36 also includes a long code PNgenerator 124 which generates a PN_(u) sequence corresponding to a PNsequence generated by the mobile unit in the mobile-to-cell link. PNgenerator 124 can be a maximal-length linear sequence generator thatgenerates a user PN code that is very long, for example degree 42, timeshifted in accordance with an additional factor such as the mobile unitaddress or user ID to provide discrimination among users. Thus thecell-site received signal is modulated by both the long code PN_(u)sequence and the short code PN_(I) and PN_(Q) sequences. In thealternative, a non-linear encryption generator, such as an encryptorusing the data encryption standard (DES) to encrypt a 64-symbolrepresentation of universal time using a user specific key, may beutilized in place of PN generator 124.

The PN_(u) sequence output from PN generator 124 is exclusive-OR'ed withthe PN_(I) and PN_(Q) sequences respectively in exclusive-OR gates 126and 128 to provide the sequences PN_(I') and PN_(Q').

The sequences PN_(I') and PN_(Q') are provided to PN QPSK correlator 130along with the I and Q channel signals output from receiver 32.Correlator 130 is utilized to correlate the I and Q channel data withthe PN_(I') and PN_(Q') sequences. The correlated I and Q channeloutputs of correlator 130 are respectively provided to accumulators 132and 134 where the symbol data is accumulated over a 4-chip period. Theoutputs of accumulators 132 and 134 are provided as inputs to FastHadamard Transform (FHT) processor 136. FHT processor 148 produces a setof 64 coefficients for every 6 symbols. The 64coefficients are thenmultiplied by a weighing function generated in control processor 48. Theweighing function is linked to the demodulated signal strength. Theweighted data output from FHT 136 is provided to diversity combiner anddecoder circuitry 50 (FIG. 14) for further processing.

The second receiver system processes the received signals in a mannersimilar to that discussed with respect to the first receiver system ofFIGS. 14 and 15. The weighted 64 symbols output from receivers 36 and 46are provided to diversity combiner and decoder circuitry 40. Circuitry50 includes an adder which adds the weighted 64 coefficients fromreceiver 36 to the weighted 64 coefficients from receiver 46. Theresulting 64 coefficients are compared with one another in order todetermine the largest coefficient. The magnitude of the comparisonresult, together with the identity or the largest of the 64coefficients, is used to determine a set of decoder weights and symbolsfor use within a Viterbi algorithm decoder implemented in circuitry 50.

The Viterbi decoder contained within circuitry 50 is of a type capableof decoding data encoded at the mobile unit with a constraint lengthK=9, and of a code rate r=1/2. The Viterbi decoder is utilized todetermine the most likely information bit sequence. Periodically,nominally 1.25 msec, a signal quality estimate is obtained andtransmitted as a mobile unit power adjustment command along with data tothe mobile unit. Further information on the generation of this qualityestimate is discussed in further detail in U.S. Pat. No. 5,056,109entitled "Method and Apparatus for Controlling Transmission Power In ACDMA Cellular Mobile Telephone System." This quality estimate is theaverage signal-to-noise ratio over the 1.25 msec interval.

Each data receiver tracks the timing of the received signal it isreceiving. This is accomplished by the well known technique ofcorrelating the received signal by a slightly early local reference PNand correlating the received signal with a slightly late local referencePN. The difference between these two correlations will average to zeroif there is no timing error. Conversely, if there is a timing error,then this difference will indicate the magnitude and sign of the errorand the receiver's timing is adjusted accordingly.

The cell-site further includes antenna 62 which is coupled to GPSreceiver 64. GPS receiver processes signals received on antenna 62 fromsatellites in the Navistar Global Positioning System satellitenavigation system so as to provide timing signals indicative ofUniversal Coordinated Time (UTC). GPS receiver 64 provides these timingsignals to control processor 48 for timing synchronizing at thecell-site as discussed previously.

In FIG. 14 optional digital data receiver 38 may be included forimproved performance of the system. The structure and operation of thisreceiver is similar to that described with reference to the datareceivers 36 and 46. Receiver 38 may be utilized at the cell-site toobtain additional diversity modes. This additional data receiver aloneor in combination with additional receivers can track and receive otherpossible delay paths of mobile unit transmitted signals. Optionaladditional digital data receivers such as receiver 38 providesadditional diversity modes which are extremely useful in thosecell-sites which are located in dense urban areas where manypossibilities for multipath signals occur.

Signals from the MTSO are coupled to the appropriate transmit modulatorvia digital link 52 under control of control processor 48. Transmitmodulator 54 under control of control processor 48 spread spectrummodulates the data for transmission to the intended recipient mobileunit.

The output of transmit modulator 54 is provided to transmit powercontrol circuitry 56 where under the control of control processor 48 thetransmission power may be controlled. The output of circuitry 56 isprovided to summer 57 where it is summed with the output of transmitmodulator/transmit power control circuits directed to other mobiles inthe cell. The output of summer 57 is provided to transmit poweramplifier circuitry 58 where output to antenna 60 for radiating tomobile units within the cell service area. FIG. 14 further illustratespilot/control channel generators and transmit power control circuitry66. Circuitry 66 under control of control processor generates and powercontrols the pilot signal, the sync channel, and the paging channel forcoupling to circuitry 58 and output to antenna 60.

FIG. 16 illustrates in block diagram form an exemplary mobile unit CDMAtelephone set. The mobile unit CDMA telephone set includes an antenna430 which is coupled through diplexer 432 to analog receiver 434 andtransmit power amplifier 436. Antenna 430 and diplexer 432 are ofstandard design and permit simultaneous transmission and receptionthrough a single antenna. Antenna 430 collects transmitted signals andprovides them through diplexer 432 to analog receiver 434. Receiver 434receives the RF frequency signals from diplexer 432 which are typicallyin the 850 MHz frequency band for amplication and frequencydownconversion to an IF frequency. This translation process isaccomplished using a frequency synthesizer of standard design whichpermits the receiver to be tuned to any of the frequencies within thereceive frequency band of the overall cellular telephone frequency band.The signals are also filtered and digitized for providing to digitaldata receivers 440 and 442 along with searcher receiver 444.

The details of receiver 434 are further illustrated in FIG. 17. Receivedsignals from antenna 430 are provided to downcoverter 500 which iscomprised of RF amplifier 502 and mixer 504. The received signals areprovided as an input to RF amplifier 502 where they are amplified andoutput as an input to mixer 504. Mixer 504 is provided with anotherinput, that being the signal output from frequency synthesizer 506. Theamplified RF signals are translated in mixer 504 to an IF frequency bymixing with the frequency synthesizer output signal.

The IF signals are output from mixer 504 to bandpass filter (BPF) 508,typically a Surface Acoustic Wave (SAW) filter having a passband ofapproximately 1.25 MHz, where they are from bandpass filtered. Thecharacteristics of the SAW filter are chosen to match the waveform ofthe signal transmitted by the cell-site. The cell-site transmittedsignal is a direct sequence spread spectrum signal that is modulated bya PN sequence clocked at a predetermined rate, which in the exemplaryembodiment is 1.2288 MHz. This clock rate is chosen to be an integermultiple of the baseband data rate of 9.6 kbps.

The filtered signals are output from BPF 508 as an input to a variablegain IF amplifier 510 where the signals are again amplified. Theamplified IF signals are output from IF amplifier 510 to analog todigital (A/D) converter 512 where the signals are digitized. Theconversion of the IF signal to a digital signal occurs at a 9.8304 MHzclock rate in the exemplary embodiment which is exactly eight times thePN chip rate. Although (A/D) converter 512 is illustrated as part ofreceiver 534, it could instead be a part of the data and searcherreceivers. The digitized IF signals are output from (A/D) converter 512to data receivers 440 and 442, and searcher receiver 444.

Receiver 434 also performs a power control function for adjusting thetransmit power of the mobile unit. An automatic gain control (AGC)circuit 514 is also coupled to the output of IF amplifier 510. Inresponse to the level of the amplified IF signal, AGC circuit 514provides a feedback signal to the gain control input of IF amplifier510. Receiver 434 also uses AGC circuit 514 to generate an analog powercontrol signal that is provided to transmit power control circuitry 438.

In FIG. 16, the digitized signal output from receiver 434 is provided todigital data receivers 440 and 442 and to searcher receiver 444. Itshould be understood that an inexpensive, low performance mobile unitmight have only a single data receiver while higher performance unitsmay have two or more to allow diversity reception.

The digitized IF signal may contain the signals of many on-going callstogether with the pilot carriers transmitted by the current cell-siteand all neighboring cell-sites. The function of the receivers 440 and442 are to correlate the IF samples with the proper PN sequence. Thiscorrelation process provides a property that is well-known in the art as"processing gain" which enhances the signal-to-interference ratio of asignal matching the proper PN sequence while not enhancing othersignals. Correlation output is then synchronously detected using thepilot carrier from the closest cell-site as a carrier phase reference.The result of this detection process is a sequence of encoded datasymbols.

A property of the PN sequence as used in the present invention is thatdiscrimination is provided against multi-path signals. When the signalarrives at the mobile receiver after passing through more than one path,there will be a difference in the reception time of the signal. Thisreception time difference corresponds to the difference in distancedivided by the velocity of propagation. If this time difference exceedsone microsecond, then the correlation process will discriminate betweenone of the paths. The receiver can choose whether to track and receivethe earlier or later path. If two receivers are provided, such asreceivers 440 and 442, then two independent paths can be tracked andprocessed in parallel.

Searcher receiver 444, under control of control processor 446 is forcontinuously scanning the time domain around the nominal time of areceived pilot signal of the cell-site for other multipath pilot signalsfrom the same cell-site and for other cell-site transmitted pilotsignals. Receiver 444 will measure the strength of any reception of adesired waveform at times other than the nominal time. Receiver 444compares signal strength in the received signals. Receiver 444 providesa signal strength signal to control processor 446 indicative of thestrongest signals.

Processor 446 provides control signals to data receivers 440 and 442 foreach to process a different one of the strongest signals. On occasionanother cell-site transmitted pilot signal is of greater signal strengththan the current cell-site signal strength. Control processor 446 thenwould generate a control message for transmission to the systemcontroller via the current cell-site requesting a transfer of the cellto the cell-site corresponding to the strongest pilot signal. Receivers440 and 442 may therefor handle calls through two different cell-sites.

During a soft handoff operation, the mobile unit will be receivingsignals from two or more cells. Because the mobile unit can only alignits timing in response to one of cells' timing adjust commands, themobile unit will normally move its timing in response to the commandsreceived from the strongest cell being received. The mobile unittransmitted signal will thus be in time alignment with the cell withwhich it has the best path. Otherwise greater mutual interference toother users will result.

Further details of an exemplary receiver, such as data receiver 440 isillustrated in further detail in FIG. 17. Data receiver 440 includes PNgenerators 516 and 518 which generate the PN_(I) and PN_(Q) sequences ina manner and corresponding to those generated by the cell-site. Timingand sequence control signals are provided to PN generators 516 and 518from control processor 446. Data receiver 440 also includes Walshgenerator 520 which provides the appropriate Walsh function forcommunication with this mobile unit by the cell-site. Walsh generator520 generates, in response to timing signals (not shown) and a functionselect signal from the control processor, a signal corresponding to anassigned Walsh sequence. The function select signal transmitted to themobile unit by the cell-site as part of the call set up message. ThePN_(I) and PN_(Q) sequences output from PN generators 516 and 518 arerespectively input to exclusive-OR gates 522 and 524. Walsh generator520 provides its output to both of exclusive-OR gates 522 and 524 wherethe signals are exclusive-OR'ed and output the sequences PN_(I') andPN_(Q').

The sequences PN_(1') and PN_(Q') are provided to receiver 440 wherethey are input to PN QPSK correlator 526. PN correlator 526 may beconstructed in a manner similar to the PN correlator of the cell-sitedigital receivers. PN correlator 526 correlates the received I and Qchannel data with the PN_(I') and PN_(Q') sequences and providescorrelated I and Q channel data output to corresponding accumulators 528and 530. Accumulators 528 and 530 accumulate the input information overa period of one symbol or 64 chips. The accumulator outputs are providedto phase rotator 532 which also receives a pilot phase signal fromcontrol processor 446. The phase of the received symbol data is rotatedin accordance with the phase of the pilot signal as determined by thesearcher receiver and the control processor. The output from phaserotator 532 is the I channel data which is provided to the deinterleaverand decoder circuitry.

Control processor 446 also includes PN generator 534 which generates theuser PN sequence in response to an input mobile unit address or user ID.The PN sequence output from PN generator 534 is provided to diversitycombiner and decoder circuitry. Since the cell-to-mobile signal isscrambled with the mobile user address PN sequence, the output from PNgenerator 534 is used in descrambling the cell-site transmitted signalintended for this mobile user similar to that as in the cell-sitereceiver. PN generator 534 specifically provides the output PN sequenceto the deinterleaver and decoder circuitry where it is used todescramble the scrambled user data. Although scrambling is discussedwith reference to a PN sequence, it is envisioned that other scramblingtechniques including those well known in the art may be utilized.

The outputs of receivers 440 and 442 are thus provided to diversitycombiner and decoder circuitry 448. The diversity combiner circuitrycontained within circuitry 448 simply adjusts the timing of the twostreams of received symbols into alignment and adds them together. Thisaddition process may be proceeded by multiplying the two streams by anumber corresponding to the relative signal strengths of the twostreams. This operation can be considered a maximal ratio diversitycombiner. The resulting combined signal stream is then decoded using aforward error detection (FEC) decoder also contained within circuitry448. The usual digital baseband equipment is a digital vocoder system.The CDMA system is designed to accommodate a variety of differentvocoder designs.

Baseband circuitry 450 typically includes a digital vocoder (not shown)which may be a variable rate type as disclosed in patent applicationSer. No. 07/713,661, filed Jun. 11, 1991. Baseband circuitry 450 furtherserves as an interface with a handset or any other type of peripheraldevice. Baseband circuitry 450 accommodates a variety of differentvocoder designs. Baseband circuitry 450 provides output informationsignals to the user in accordance with the information provided theretofrom circuitry 448.

In the mobile-to-cell link, user analog voice signals are typicallyprovided through a handset as an input to baseband circuitry 450.Baseband circuitry 450 includes an analog to digital (A/D) converter(not shown) which converts the analog signal to digital form. Thedigital signal is provided to the digital vocoder where it is encoded.The vocoder output is provided to a forward error correction (FEC)encoding circuit (not shown) for error correction. In the exemplaryembodiment the error correction encoding implemented is of aconvolutional encoding scheme. The digitized encoded signal is outputfrom baseband circuitry 450 to transmit modulator 452.

Transmit modulator 452 first Walsh encodes the transmit data and thenmodulates the encoded signal on a PN carrier signal whose PN sequence ischosen according to the assigned address function for the call. The PNsequence is determined by control processor 446 from call setupinformation that is transmitted by the cell-site and decoded byreceivers 440 and 442 and control processor 446. In the alternative,control processor 446 may determine the PN sequence throughprearrangement with the cell-site. Control processor 446 provides the PNsequence information to transmit modulator 452 and to receivers 440 and442 for call decoding.

The output of transmit modulator 452 is provided to transmit powercontrol circuitry 438. Signal transmission power is controlled by theanalog power control signal provided from receiver 434. Control bitstransmitted by the cell-sites in the form power adjustment command areprocessed by data receivers 440 and 442. The power adjustment command isused by control processor 446 in setting the power level in mobile unittransmission. In response to this command, control processor 446generates a digital power control signal that is provided to circuitry438. Further information on the relationship of receivers 440 and 442,control processor 446 and transmit power control 438 with respect topower control is further described in the above-mentioned patent.

Transmit power control circuitry 438 outputs the power controlledmodulated signal to transmit power amplifier circuitry 436. Circuitry436 amplifies and converts the IF signal to an RF frequency by mixingwith a frequency synthesizer output signal which tunes the signal to theproper output frequency. Circuitry 436 includes an amplifier whichamplifies the power to a final output level. The intended transmissionsignal is output from circuitry 436 to diplexer 432. Diplexer 432couples the signal to antenna 430 for transmission to the cell-sites.

Control processor 446 also is capable of generating control messagessuch as cell-diversity mode requests and cell-site communicationtermination commands. These commands are provided to transmit modulator452 for transmission. Control processor 446 is responsive to the datareceived from data receivers 440, 442 and search receiver 444 for makingdecisions relative to handoff and diversity combining.

Furthermore, it is to be understood that although the present inventionhas been described with reference to a preferred embodiment, variousmodifications, known to those skilled in the art, may be made to thestructures and process steps presented herein without departing from theinvention as recited in the several claims appended hereto.

What is claimed is:
 1. A method for determining the position of a mobilestation within a cellular telephone system having first and second basestations, comprising the steps of:(A) measuring, at said first basestation, a round trip signal propagation time between said first basestation and said mobile station; (B) transmitting a first signal fromsaid first base station to said mobile station, and transmitting asecond signal from said second base station to said mobile station; (C)measuring, at said mobile station, an arrival time differencerepresenting a time interval between a first relative time when saidfirst signal is received at said mobile station and a second relativetime when said second signal is received at said mobile station; and (D)determining said position of said mobile station in accordance with saidround trip propagation time and said arrival time difference.
 2. Themethod of claim 1, wherein said first base station includes a sectorantenna, and step (D) further comprises determining, with said sectorantenna, a sector in which said mobile station is positioned, and saidposition of said mobile station is determined in step (D) in accordancewith said round trip propagation time, said arrival time difference andsaid sector.
 3. The method of claim 2, wherein said round trippropagation time is used in step (D) to calculate a circle on which saidmobile station is positioned, said arrival time difference is used toidentify first and second candidate positions of said mobile stationalong said circle, and said sector is used for selecting said positionof said mobile station from said first and second candidate positions.4. The method of claim 1, further comprising the step of using a mapmatching table to estimate said position of said mobile station fromsaid round trip propagation time and said arrival time difference. 5.The method of claim 1, wherein step (D) is performed within a switchingcenter of said cellular system.
 6. The method of claim 1, wherein step(D) is performed with said first base station.
 7. A method fordetermining the position of a mobile station within a cellular telephonesystem having first and second base stations, comprising the stepsof:(A) measuring, at said first base station, a first round trip signalpropagation time between said first base station and said mobilestation; (B) measuring, at said second base station, a second round tripsignal propagation time between said second base station and said mobilestation; and (C) determining said position of said mobile station inaccordance with said first and second round trip propagation times. 8.The method of claim 7, wherein said first base station includes a sectorantenna, and step (C) further comprises determining, with said sectorantenna, a sector in which said mobile station is positioned, and saidposition of said mobile station is determined in accordance with saidfirst and second round trip propagation times and said sector.
 9. Themethod of claim 8, wherein said first round trip propagation time isused in step (C) to calculate a first circle on which said mobilestation is positioned, said second round trip propagation time is usedto calculate a second circle on which said mobile station is positioned,said first and second circles intersecting at first and second candidatepositions of said mobile station, wherein said sector is used forselecting said position of said mobile station from said first andsecond candidate positions.
 10. The method of claim 7, furthercomprising the step of using a map matching table to estimate saidposition of said mobile station from said first and second said roundtrip propagation times.
 11. The method of claim 7, wherein step (C) isperformed within a switching center in said cellular system.
 12. Themethod of claim 7, wherein step (C) is performed with said first basestation.
 13. The method of claim 7, further comprising the step of,prior to steps (A) and (B), temporarily transmitting a signal at anincreased power from said mobile station, and measuring said first andsecond round trip signal propagation times in steps (A) and (B) usingsaid signal transmitted at said increased power level.
 14. The method ofclaim 13, further comprising the step of, after steps (A) and (B),transmitting said signal at a lower power level that is less than saidincreased power from said mobile station.